The present invention relates to an arrangement for the production of x-ray sectional pictures, comprising an x-ray source for the production of x-rays which penetrate the object in at least one sectional plane, an x-ray screen which serves to convert penetrated x-rays into radiation to which a provided recording medium is sensitive, a focusing optics by means of which each point of the screen is portrayed in the form of a line on the recording medium, where points located beside one another are portrayed as lines located beside one another, and a rotation device by means of which the object and the x-ray source with the screen may be rotated relative to one another about an axis of rotation which is at right angles to the sectional plane.
Arrangements for the production of x-ray sectional pictures of the type referred to in the introduction are known (Peters, T. M.: Spatial Filtering to Improve Transverse Tomography, IEEE Transactions BME-21, No. 3, May 1974, pages 214-219). In this arrangement the x-ray source consists of a conventional x-ray tube and a slotted diaphragm composed of lead. The lead absorbs the oncoming x-rays so that the latter can only emerge from the slot and thus possess an areal distribution. These x-rays irradiate an object, for example a human body, and the x-rays which have penetrated through are fed to an x-ray screen by which they are converted into radiation to which a provided recording medium is sensitive, for example ultraviolet light (UV-light) or visible light. Due to the areal distribution of the x-rays only a narrow layer, a so-called sectional plane, of the object is x-rayed and therefore only a narrow strip running parallel to the sectional plane is recorded on the screen. This strip is focused onto a photo-plate via a focusing optics. This focusing optics has the property of portraying a point on the excited strip of the screen in the form of a line onto the photo-plate, and points lying beside one another are portrayed as lines lying beside one another. Here points lying beside one another are those which do not lie on a straight line running at right angles to the excited strip in the image plane. The focusing optics which carries out this function consists, in the known arrangement, of a cylindrical lens and a spherical lens, both of which are arranged in the beam path between the screen and the photo-plate. In accordance with the quoted publication, in order to achieve a fan beam geometry, the cylindrical lens must be replaced by a conical lens.
Arrangements of this type are preferably used in x-ray diagnosis, in which the associated processes for the production of x-ray sectional pictures are known under the name "Transaxial tomography". These processes facilitate the representation of the density distribution in a sectional plane through the object--the patient--without the covering and overlapping of adjacent parts which are unavoidable in conventional processes.
The sectional picture itself is obtained from the projection data formed when the relevant cross sectional plane is irradiated in various directions. A first step consists in the back-projection and superimposition of the projection data. Here a so-called layergram or Heckmann image is formed which is a representation of the desired density distribution smeared with the function 1/r. Here smearing is to be understood in that an ideal point is not reproduced as an ideal point but as a spot. The intensity distribution of this spot is described by the so-called point spread function (psf). Smearing with the function 1/r is to be understood in that the psf decreases from the center outwards with the function 1/r.
The desired density distribution is obtained from the "layergram" in that the 1/r smearing is cancelled. This is carried out by means of a special local frequency filtering which is known under the slogan "rho filtering". This "rho filtering" is characterized by a modulation transmission function which, plotted over the spatial frequency from 0, possesses an ascent which is to be linear as far as possible. For the "rho filtering", the developed image of the layergram is processed either analog-optically or digital-electronically (see above quoted publication). This step of the filtering of the layergram is time consuming and technically elaborate however.